In order to reduce weight and increase structural strength, aircraft are increasingly using composite materials, which often are non-conductive or partially conductive. Aircraft also have to manage electrical currents caused by electromagnetic effects (EME) such as those caused by lightning, radio towers, and static discharge (e.g., air caused) by providing conductive paths and electrical isolation where required. EME effects such as lightning may have large currents and high voltages capable of producing electrical arcing across exposed surfaces. In aircraft made with partially or non-conductive composite materials, good electrical paths are required to dissipate EME energy, and good electrical isolation is required to protect sensitive equipment and fluids. Bonding and grounding requirements for aircraft electrical systems such as wing ice protection systems must also be provided for.
Leading edge structures such as slats with track arms and other aircraft structures are generally designed to have grounding paths to dissipate EME energy. However, for normal operation, the electrical system has a current return network for grounding the electrical system. It may be desirable that the electrical system conduct ground current through the current return network during normal operation and not through the aircraft structure ground paths used for EME effects.
A structural connection of a leading edge slat to a track arm may utilize bearings with a lubricant that is non-conductive or partially conductive to normal ground currents. This provides the resistance to normal grounding currents, while accommodating EME effects with sufficient power to break though the resistive barrier of the non-conducting lubricant. In such a structure, a ground path through a bearing may have resistance to grounding currents, but areas around the bearing lubricant could be places where sufficiently high voltages and currents may cause electrical arcing through air gaps around the metal bearing parts. One existing solution is to include attachment points on the structures to incorporate flexible electrical jumpers to provide an electrical ground path. However, this requires additional cost, design, assembly, maintenance checks, and added weight to the aircraft. Another existing solution is to paint sealant around exposed bearing parts. The reliability of this procedure may be less than desired, and sealant may not be compliant enough to allow a bearing to function properly during cold temperatures.
Therefore, there is a need for bearings that provide reliable, low cost, easy-to-maintain isolation from normal electrical currents and voltages, while accommodating overflow currents from EME effects.